Method for homogenizing glass

ABSTRACT

A method for homogenizing glass includes the method: providing a cylindrical blank composed of the glass having a cylindrical outer surface that extends along a longitudinal axis of the blank between a first end face and a second end face, forming a shear zone in the blank by softening a longitudinal section of the blank and subjecting it to a thermal-mechanical intermixing treatment, and displacing the shear zone along the longitudinal axis of the blank. The displacement of the shear zone along the longitudinal axis of the blank is superimposed by a simultaneous oscillating motion of the shear zone along the longitudinal axis of the blank. The first end of the blank is rotated at a first rotational speed and the second end of the blank is rotated at a second rotational speed. An oscillating motion of the shear zone is generated by periodically varying the first and/or second rotational speed.

CROSS-REFERENCED TO RELATED APPLICATION

This Utility patent application is a Continuation patent Application ofU.S. Ser. No. 16/662,667 filed Oct. 24, 2019, which claims priority toEuropean Application No. 18 202 857.1 filed on Oct. 26, 2018, both ofwhich are incorporated herein by reference. This Utility patentapplication is related to co-pending U.S. Ser. No. 16/662,610, entitled“METHOD AND DEVICE FOR HOMOGENIZING GLASS”, filed Oct. 24, 2019.

TECHNICAL FIELD

One aspect relates to a method for homogenizing glass by providing acylindrical blank composed of the glass having a cylindrical outersurface that extends along a longitudinal axis of the blank over alength of the blank between a first end face and a second end face,forming a shear zone in the blank by softening a longitudinal section ofthe blank and subjecting it to a thermal-mechanical intermixingtreatment, and displacing the shear zone along the longitudinal axis ofthe blank.

BACKGROUND

A zone melting method of this type is known from U.S. Pat. No. 3,485,613A. The solid glass cylinder or the glass cylinder filled with a powdermixture, which is clamped in a glass lathe, is locally heated and istwisted zonally. As the heat source, single- or multi-flame burners orelectric heat sources are employed. The dimension of the shear zone inthe direction of the axis of rotation (=width of the shear zone) dependson the viscosity. For viscosities of less than about 10¹³ Poise (dPa·s),it is adjusted to a value in the range of between 0.1 and 3 times therod diameter, and for viscosities of less than about 10⁵ Poise (dPa·s)to a value in the range of between 0.1 and 1 times the rod diameter. Itcan be narrowed by laterally acting cooling means.

The blank to be homogenized is arranged horizontally and the supportsfor the ends of the blank are horizontally opposite each other. Theshear zone produced on the basis of this concept is theoretically even,circular and rotationally symmetrical. The material transport in theshear zone runs substantially tangentially, but not in the radialdirection. This means that radial glass defects in the blank are verydifficult or impossible to eliminate. In particular, any bubbles thatare present are not transported to the cylindrical outer surface, butthey remain in the bulk of the material. Equally, radial concentrationgradients of a dopant are not eliminated. The problem is intensified bythe fact that heat transport from the outside to the inside is slow as aresult of the thermally insulating effect of quartz glass. It istherefore colder in the centre of the shear zone than at the surface,which contributes to a higher viscosity and lower intermixing and makesit difficult for any crystallites to melt completely. Also, it isimpossible to achieve homogenization in the direction of the axis ofrotation by a simple zone melting method.

To homogenize a quartz glass composition in three directions runningperpendicular to each other, a multi-step zone melting method isproposed in EP 673 888 B1, in [which] a ball-like quartz glass body isproduced as an intermediate product by compressing a twisted rod, atboth ends of which support rods are placed, which run transversely to aprevious axis of rotation and by means of which the ball-like quartzglass body is elongated and subjected to a further zone melting methodwith a different axis of rotation. During twisting, one support rodrotates at e.g. 70 to 100 revolutions per minute and the other supportrod rotates in the opposite direction at 1 to 3 times this speed.Oxyhydrogen or propane gas burners or electric heating elements areemployed as the heat source.

U.S. Pat. No. 2,904,713 A describes a homogenizing process for quartzglass, in which the softened quartz glass composition supported on twosupport rods is alternately compressed and stretched by moving thesupport tubes away from or towards each other.

The known multi-step zone melting method is time-consuming andenergy-intensive.

One aspect is therefore based on the problem of modifying the zonemelting method for homogenizing glass, in particular for glass with ahigh Sift content and particularly for quartz glass, such that with thelowest possible time and energy input, in addition to the tangentialmixing a radial mixing within the shear zone is made possible.

SUMMARY

One embodiment relates to a method for homogenizing glass, comprisingthe following steps:

-   -   (a) providing a cylindrical blank composed of the glass having a        cylindrical outer surface that extends along a longitudinal axis        of the blank over a length of the blank between a first end face        and a second end face,    -   (b) forming a shear zone in the blank by softening a        longitudinal section of the blank and subjecting it to a        thermal-mechanical intermixing treatment, and    -   (c) displacing the shear zone along the longitudinal axis of the        blank

Optical components composed of glass installed in high-precision systemsare subject to strict requirements in terms of their transparency andhomogeneity. Often, however, glass exhibits heterogeneous structures,such as layers and so-called “striae”, which are attributable to regionsof glass with different compositions or differences in the refractiveindex.

This is particularly problematic for high-silica glass with a highcontent of Sift of e.g. more than 80 wt. %, and in particular for quartzglass with an Sift content of 87 wt. % or more. In this case, even attemperatures close to sublimation the viscosity can still be so highthat homogenization in a crucible by stirring or refining is impossible.

To eliminate striae and layers in quartz glass, crucible-free meltingmethods are known in which a cylindrical starting body is clamped inheadstocks of a glass lathe and softened zonally, the headstockssimultaneously rotating at different speeds or in opposite directionsaround an axis of rotation. As a result of the different rotation of thestarting body on either side of the softening zone, torsion (twisting)occurs there and thus mechanical intermixing in the bulk of the glass.The region of the intermixing is also referred to here as the “shearzone”. The shear zone is displaced along the starting body and this isshaped and intermixed along its length in the process. Heterogeneousstructures (striae and layers) are thus reduced or eliminated. Theresult of this thermal-mechanical intermixing treatment is a blankcomposed of at least partially homogenized glass. This type ofthermal-mechanical intermixing treatment by tool-free shaping is alsoreferred to below as a “homogenizing process”, “zone melting method” or“twisting”, and the at least partially homogenized cylindrical blankthat is present after the twisting is referred to as a “twisted rod”.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of embodiments and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments andtogether with the description serve to explain principles ofembodiments. Other embodiments and many of the intended advantages ofembodiments will be readily appreciated as they become better understoodby reference to the following detailed description. The elements of thedrawings are not necessarily to scale relative to each other. Likereference numerals designate corresponding similar parts.

Embodiments are described in more detail below with reference to anexemplary embodiment and a drawing. The individual figures illustrateschematic illustrations of the following:

FIG. 1 : a device for carrying out the zone melting method with axes ofrotation offset relative to each other according to one embodiment,

FIG. 2 : a rotation figure for a blank which is obtained in a firstprocedure,

FIG. 3 : a rotation figure for a blank which is obtained in a secondprocedure,

FIG. 4 : a path-time diagram for the translational speed of a heatsource along the longitudinal axis of the blank in a third procedureaccording to one embodiment,

FIG. 5 : a variant of the zone melting method with axes of rotationtilted relative to each other according to one embodiment,

FIG. 6 : measures for keeping the tilt angle constant throughout atwisting stroke in the procedure of FIG. 5 ,

FIG. 7 : an enlarged illustration of a tubular thermal radiationdissipator in a view of its end face,

FIG. 8 : a diagram to explain the action of an embodiment of thethermal-mechanical intermixing treatment on the radial OH-groupconcentration profile in the treated blank,

FIG. 9 : a diagram to explain the action of a further embodiment of thethermal-mechanical intermixing treatment on the radial OH-groupconcentration profile in the treated blank,

FIG. 10 : a sketch to explain an embodiment of the thermal-mechanicalintermixing treatment and

FIG. 11 : a schematic illustration of a means suitable for carrying outthe thermal-mechanical intermixing treatment.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which isillustrated by way of illustration specific embodiments in which oneembodiments may be practiced. In this regard, directional terminology,such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc.,is used with reference to the orientation of the Figure(s) beingdescribed. Because components of embodiments can be positioned in anumber of different orientations, the directional terminology is usedfor purposes of illustration and is in no way limiting. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope of thepresent embodiments. The following detailed description, therefore, isnot to be taken in a limiting sense, and the scope of the presentembodiments are defined by the appended claims.

It is to be understood that the features of the various exemplaryembodiments described herein may be combined with each other, unlessspecifically noted otherwise.

According to one embodiment, starting from a method of the typementioned above, problems are solved on the one hand by the fact thatcylindrical sections of the blank are adjacent to both ends of the shearzone, of which the first cylindrical section has a first central axisand the second cylindrical section has a second central axis, the firstcentral axis and the second central axis being at least temporarilynon-coaxial with each other.

The method according to one embodiment is used for the production ofglass, in particular of high-silica glass and most particularly of pureor doped quartz glass, which is at least partially homogenized. Thecylindrical blank here is subjected to a thermal-mechanical intermixingtreatment, the zone melting method. To this end, the blank—generallyextended at both ends by means of fused-on support rods—is clamped intoa rotating means, such as a glass lathe, which is equipped with at leastone heat source for the local softening of the blank. As a result ofdifferent rotational speeds and/or directions of rotation of the blanksupports at either end, a shear zone forms in the softened glass, inwhich torsion and thermal-mechanical intermixing of the glass takeplace. By continuously displacing the heating means along the blankand/or by continuously displacing the blank along the heating means, theshear zone is pushed through the blank. This thermal-mechanicalintermixing treatment includes one or more passes (multiple twistingstrokes), in which the shear zone is displaced along the longitudinalaxis of the blank in one direction and/or in a reversing manner.

The spatial orientation of the longitudinal axis of the blank duringhomogenizing is arbitrary. The blank is often clamped in the rotatingmeans with a horizontally oriented longitudinal axis, in which casedummy rods can be welded to the ends of the blank to minimise the lossof good material. The blank holders at both ends, such as e.g. thespindles of a glass lathe, have the same axis of rotation in the priorart and this is coaxial with the longitudinal axis of the blank to behomogenized. A circular shear zone forms, wherein the central axes ofthe cylindrical sections of the blank bordering both ends of the shearzone are coaxial with each other and in the common axis of rotation.

In contrast thereto, it is provided that the first central axis and thesecond central axis are at least temporarily non-coaxial with eachother. The shear zone that forms as a result is not rotationallysymmetrical, i.e. also not circular. It has been shown that the materialtransport within this shear zone takes place not only purelytangentially but also in the radial direction.

This non-coaxial orientation of the central axes of the two cylindricalsections is established most simply in a blank with a shear zone that isalready softened; it can be maintained permanently during thethermal-mechanical intermixing treatment or it can be varied in time orextent during said treatment. The effect of a “three-dimensionalhomogenizing” according to the prior art in terms of freedom fromstriae, freedom from bubbles and refractive index distribution is, to acertain extent, achievable in this way even with only a singleintermixing operation (twisting stroke).

The zone melting method with non-coaxial central axes can be implementedin various ways. For example, the central axes of the two cylindricalsections can be offset parallel to each other, they can be tiltedrelative to each other, thus forming an angle, or they can run skew toeach other.

In a first basic concept, the first central axis and the second centralaxis are at least temporarily offset relative to each other. In thesimplest case the first central axis and the second central axis runparallel (in their extension) but with a lateral offset relative to eachother.

The offset of the central axes here is in one embodiment adjusted to avalue in the range of between 0.5% and 15% of the diameter of the blank.For blanks with outer diameters of less than 50 mm, a central axisoffset in the upper offset range (2 to 15%) is most suitable, and forblanks with outer diameters of 50 mm and more a central axis offset inthe lower offset range (0.5 to 7%) is most suitable.

In a preferred embodiment of this basic concept, the first central axisand second central axis are offset relative to each other, at least thefirst central axis additionally being offset relative to a first machineaxis of rotation around which it is rotated.

The first machine axis of rotation corresponds e.g. to the axis ofrotation of the rotating means for the first cylindrical section. Thefirst central axis of the cylindrical section generally runs parallel tothis axis of rotation but with a lateral offset, i.e. outside the axisof rotation of the rotating means. The relevant cylindrical section heredoes not necessarily rotate around its own (first) central axis, butaround the first machine axis of rotation, so that during this rotationit describes a circular path around this first machine axis of rotation.Since this axis of rotation does not run through the centre of gravityof the first cylindrical section, a static imbalance forms whichpermanently deforms the shear zone.

Similarly, the central axis of the second cylindrical section can inturn be offset relative to a second machine axis of rotation, whichcorresponds e.g. to the axis of rotation of the rotating means for thesecond cylindrical section. In one embodiment, however, the secondcylindrical section rotates around its own (second) central axis, whichis coaxial with the second machine axis of rotation and particularly inone embodiment also coaxial with the first machine axis of rotation.

FIG. 10 explains this concept with the aid of a position and rotationsketch. The first cylindrical section 1.1 here is connected to a firstspindle 6 (see FIG. 1 ) of a lathe and the second cylindrical section1.2 to a second spindle 7 of the lathe. The spindles 6; 7 at either enddefine a machine axis of rotation 104. In the exemplary embodiment thesecond cylindrical section 1.2 is mounted eccentrically on the secondspindle 7 in relation to the machine axis of rotation; in other words,the central axis 7.1 of the second cylindrical section 1.2 lies outsidethe machine axis of rotation 104 of the lathe. The relevant cylindricalsection 1.2 therefore describes a circular path 106 around the axis ofrotation 104 with the rotational speed and direction of rotation 108 ofthe second spindle. The first cylindrical section 1.1 can be arrangedeccentrically or coaxially relative to the axis of rotation 104. In theexemplary embodiment of FIG. 10 , however, the central axis 6.1 and themachine axis of rotation 104 coincide, the first cylindrical section 6.1rotating around its central axis 6.1 by means of the first spindle inone embodiment with a direction of rotation 107 opposite to 108. Thesecond cylindrical section 1.2 additionally rotates about its centralaxis 7.1 in the direction of rotation indicated by the rotationaldirection arrow 109.

This is achieved in design terms e.g. by a device as illustratedschematically in FIG. 11 . For the eccentric support of the secondcylindrical section 1.2 here, a chuck 112 that can be displaced from themiddle of the spindle head is employed for the rotatable support of thefirst cylindrical section 1.2. Similarly, the first cylindrical section1.2 is also mounted rotatably around the machine axis of rotation 104 ona chuck 111 that can be displaced from the middle of the spindle head.The displaceability of the respective chucks 111, 112 is indicated bythe directional arrows 111 a and 112 a respectively. The block arrow Aillustrates the offset of the axis 7.1 from the axes 6.1 and 104.

In the shear zone in this concept there is torsion due not only todifferent rotations of the cylindrical sections around their respectivecentral axes, but also to the torque of the imbalance that ispermanently acting thereon. This results in particularly intensiveintermixing in the shear zone.

In another embodiment of the first basic concept, the first cylindricalsection is rotated around a first axis of rotation and the secondcylindrical section around a second axis of rotation, the first andsecond axes of rotation running parallel to each other and being offsetrelative to each other.

In this embodiment, the two cylindrical sections rotate around theirrespective central axes and they have no common axis of rotation. In theshear zone, torsion occurs not only through the different rotations ofthe cylindrical sections but also through the lateral offset of the twocentral axes relative to each other.

In the method variants of the first basic concept described above, thefirst central axis and the second central axis extend with a lateraloffset and parallel to each other. The parallel arrangement of thecentral axes is simple to achieve from a design point of view. A secondbasic concept of the embodiment is distinguished by the fact that thefirst central axis and the second central axis are tilted relative toeach other or run skew to each other at least at times during thehomogenizing process.

This results in particularly intensive intermixing in the shear zone. Inthe case of the tilt, the two central axes form a tilt angle which canbe varied in the course of the zone melting method. In one embodiment,however, the tilt angle is adjusted to the range of 145 to 175 degreesand is kept constant during the zone melting method. In the case of askew axis arrangement, a tilt angle is obtained in the projection of theaxis arrangement on to one of the planes in which one of the centralaxes extends.

For homogenizing heavy blanks in particular, a tilt in which thecylindrical sections extend obliquely downwards, starting from the shearzone, has proved advantageous.

This means that the shear zone is at the top and is supported by the twocylindrical sections at the bottom.

In another, equally preferred tilt variant, the cylindrical sections areoriented such that they extend obliquely upwards, starting from theshear zone.

This means that the shear zone is at the bottom, which is particularlyeasy to achieve because of the cylindrical sections' own weight.

In the embodiments of the first basic concept, the offset between thecylindrical sections at either end and/or the tilt of the respectivecentral axes relative to each other cause(s) an intermixing of thequartz glass in three spatial directions. Bubbles and otherinhomogeneities are thus drawn out around the axis of rotation in ascrew-like manner. In contrast to the known method, in which bubbles andother inhomogeneities are distributed in closed, coaxial rings aroundthe axis of rotation, which are difficult to collapse further, the screwshape can be drawn out further and thinned by rotation until itdissipates. This is also particularly readily achieved by a secondtwisting stroke in the opposite direction of rotation to the firsttwisting stroke.

On the other hand, the above-mentioned technical problem is also solvedaccording to one embodiment, starting from a method of the typementioned above, by the fact that the displacement of the shear zonealong the longitudinal axis of the blank is superimposed at least attimes with an oscillating motion of the shear zone along thelongitudinal axis of the blank.

Independently of the transverse displacement of the shear zone along thelongitudinal axis of the blank, the shear zone performs an alternatingmotion with a small amplitude. It has been shown that this also leads toa non-rotationally symmetrical shear zone within which materialtransport occurs not only purely tangentially but also in the radial andeven in the axial direction.

The oscillating motion of the shear zone is generated in a blank with analready softened shear zone and it can be maintained permanently duringthe homogenizing process or it can be varied in its extent over time. Bymeans of this measure too, the effect of “three-dimensionalhomogenizing” can be achieved to a certain extent in terms of freedomfrom striae, freedom from bubbles and refractive index distribution,even with just a single intermixing operation (twisting stroke).

In a first preferred technique, the oscillating motion of the shear zoneis generated by rotating the first end of the blank at a firstrotational speed and the second end of the blank at a second rotationalspeed, periodically varying the first and/or second rotational speed.

The changes to the first and second rotational speeds here are in oneembodiment such that both the level of at least one of the rotationalspeeds and the difference in speed between the first and secondrotational speeds vary periodically.

In another preferred technique the oscillating motion of the shear zoneis generated by the fact that the displacement of the shear zone alongthe longitudinal axis of the blank is caused by linear axialtranslational movement of a heat source along the longitudinal axis ofthe blank, wherein a reversing motion of the heat source is superimposedon the translational motion.

The heat source performs a type of pendulum motion, the path of themotion being composed of comparatively long distances of the forwardmotion of the shear zone at a displacement speed, regularly orirregularly interspersed with comparatively short distances of thebackward motion at the same or a different displacement speed.

In all the embodiments of the method described above, anon-rotationally-symmetrical shear zone is produced, which allowsintermixing not only in the tangential direction but also in the radialand partially also in the axial direction of the longitudinal axis ofthe blank. This also results in a more even radial temperaturedistribution in the shear zone, so that bubbles are transported to thesurface and the bubble content in the bulk of the glass is reducedoverall. Any crystallites present in the starting material of the blankalso melt completely and are eliminated in this way. In the case ofdoped quartz glass, furthermore, a more homogeneous dopant distributionis achieved and refractive index fluctuations are eliminated.

The more even radial temperature distribution in the shear zone alsopromotes the formation of a narrow shear zone. A narrow shear zonecauses more intensive intermixing of the bulk of the glass than acomparatively wider shear zone. The optimum width of the shear zonedepends on the diameter of the blank. As a rule of thumb, with a blankwith the diameter D the shear zone has a width that is less than 0.3×D.

In the zone melting method, the rotational speeds ω₁ and ω₂ on eitherside of the “shear zone” are unequal. The amount of the difference inthe rotational speeds at either end is obtained from Δω=|ω₂−ω₁|; in thecase of rotation in opposite directions, one of the rotational speedshas a negative sign. Within the shear zone a transition occurs from onerotational speed wi to the other ω₂. In the middle of the shear zone arotational speed is established which corresponds to the mean value v;⁻between the rotational speeds at either end, (v;⁻=(ω₂+ω₁)/2). The “shearzone” here is defined as that part of the bulk of the glass where, forthe axial change in the rotational speed dω/dx,/dω/dx/>0.5×|dω/dx|_(max) applies. The “width of the shear zone” isdefined as the longitudinal section in the direction of the longitudinalaxis of the blank in which the above condition is fulfilled.

The rotational speeds are determined by measuring the surface speedusing optical image processing and evaluating the movement ofirregularities close to the surface, such as e.g. bubbles.

In a shear zone that is too wide, both the local speed gradient and thelocal viscosity gradient are so low that bubbles, crystals and otherglass defects can be retained as a whole, depending on their thermalstability. For optimal dissipation, it is advantageous for one part ofthe glass defect to still be held in the colder, more viscous part,while less viscous, hotter melt flows over it on the other side where itis dissipated/distributed. If the viscosity gradient is too low, it isonly “turned” and is eventually simply entrained.

In a particularly preferred technique of one embodiment, thehomogenizing measures explained above that include a non-coaxialarrangement of the axes of rotation of the cylindrical sections ateither end of the shear zone are combined with the homogenizing measureslikewise explained above that relate to an oscillating motion of theshear zone.

In this technique for homogenizing glass, the measure in whichcylindrical sections of the blank are adjacent to the shear zone on bothsides, of which the first cylindrical section has a first central axisand the second cylindrical section has a second central axis, and thefirst central axis and the second central axis are at least temporarilynon-coaxial with each other during homogenizing, is combined with ameasure in which the displacement of the shear zone along thelongitudinal axis of the blank is superimposed at least at times with anoscillating motion of the shear zone along the longitudinal axis of theblank.

The oscillating motion of the shear zone here is in one embodimentgenerated on the basis of a method variant in which the first end of theblank is rotated at a first rotational speed and the second end of theblank is rotated at a second rotational speed, and the first and/orsecond rotational speed is periodically varied, or it is generated onthe basis of a method variant in which the displacement of the shearzone along the longitudinal axis of the blank is caused by linear axialtranslational movement of a heat source along the longitudinal axis ofthe blank, wherein a reversing motion of the heat source is superimposedon the translational motion

Another technique has proved favourable, in which a thermal radiationdissipator at least partially surrounding the shear zone is employed,the lateral dimension of which, in the direction of the longitudinalaxis of the blank, is greater than the shear zone and smaller than thelength of the blank, wherein the thermal radiation dissipator is movedalong the longitudinal axis of the blank synchronously with the shearzone.

The thermal radiation dissipator absorbs at least part of the heatenergy from the region of the shear zone by heat radiation, heatconduction or heat convection, is itself heated thereby and emits atleast part of this energy back to the blank and in particular to theshear zone as longer-wave infrared radiation. Because its lateraldimension is greater than the shear zone, however, heat energy is alsotransferred to the glass adjacent to the shear zone. As a result of theheating of the regions of the glass bulk bordering the shear zone, i.e.before and after the shear zone, the radial temperature gradient isreduced since, as a result of this pre-heating, the bulk of the glassthat is about to enter the shear zone requires a lower additional heatinput from the heat source in order to reach an adequate temperature.

The consequence of this is that the maximum temperature in theperipheral region of the blank, and thus also the temperature differencebetween the middle and the periphery of the blank, is lower than in ashaping process without a thermal radiation dissipator.

The thermal radiation dissipator thus reduces the temperature gradientsand contributes to evening out the temperature profile within the shearzone. As a result, the risk of cracking due to mechanical stresses [is]reduced.

With a view to, as far as possible, a complete capture, conversion andutilisation of the heat radiation emitted from the shear zone, however,a design of the thermal radiation dissipator in which it surrounds thecylindrical surface, in one embodiment in the form of a tube, has provedexpedient. The tube may optionally be completely or partially open atboth ends and it has a closed or largely closed tube wall. Losses ofheat energy by radiation or convection are thus reduced. In the simplestcase, the inner bore of the tube is cylindrical with a round, oval orpolygonal cross-section. It can extend coaxially to the longitudinalaxis of the blank and can be e.g. conical, or can have heterogeneity inthe axial direction, such as for instance a change in cross-section.Openings can be present in the tube wall, through which part of the heatcan be dissipated or through which an active cooling is possible toallow the heat input to be adjusted with a view to a shear zone that isas narrow as possible. The tube wall is in one piece or is composed ofmultiple tube sections joined together or of multiple other components.The heat source here is either located within the tube opening or actson the shear zone from outside, e.g. through one or more openings in thetube wall or through a longitudinal slit. A longitudinal slit in anotherwise continuous tube wall also has the advantage that mechanicalstresses due to the high temperatures and thermal expansion are avoided,which compensates for any disadvantages due to the longitudinal slit interms of the effect on the temperature homogenization in and around theshear zone.

A thermal radiation dissipator is employed, the dimension of which issmaller in the direction of the longitudinal axis of the blank than thelength of the blank, this being moved synchronously with the shear zonealong the longitudinal axis of the blank. Through the fact that thethermal radiation dissipator moves along the longitudinal axis of theblank together with the shear zone, it is ensured that the temperatureconditions in the shear zone and the adjacent regions of the bulk of theglass do not vary during the shaping process.

A thermal radiation dissipator that is short compared to the length ofthe blank furthermore ensures that the temperature at its inside ishigh, so that sublimation deposits are vitrified and cannot fall off onto the blank.

Between the thermal radiation dissipator and the blank, a clearance inthe range of 15% to 80% of the diameter of the blank is in oneembodiment established. The gap affects the temperature at the surfaceof the blank and the temperature distribution. With a comparativelylarge gap, the radiation intensity impinging on the surface of the blankis lower, but the irradiated surface region is larger because of thewider radiation angle. With a clearance of more than 80% of the diameterof the blank, a comparatively large irradiated area is obtained, whichcounteracts a narrow shear zone. With a comparatively small clearance ofless than 15% of the diameter of the blank, pressure can build up as aresult of enclosed gases, which impedes access for a burner or plasmaflame.

It has proved particularly expedient if a thermal radiation dissipatoris employed having a wall with a partially reflective inner surface,facing the shear zone, which is formed using a glass layer at least 0.1mm thick composed of a quartz glass that is transparent to infraredradiation from the NIR wavelength range. This glass layer in oneembodiment does not have any open pores in which foreign materials couldsettle, so that contamination is prevented from entering the blankduring the twisting process. The formation of the glass layer fromNIR-transparent quartz glass ensures that the reflectance of the innersurface, and thus its influence on the temperature profile in the regionof the shear zone, does not vary in the course of time as a result ofvaporised SiO₂ if this precipitates on the inner surface as an SiO₂layer (also referred to below as an “SiO₂ deposit”) and vitrifies as aresult of the high temperatures during the twisting process. For botheffects (purity and reflectance), a layer thickness of e.g. 0.1 mm ormore is sufficient.

The glass layer is transparent to infrared radiation from the NIRwavelength range, but part of this is reflected on the inner surfaceowing to the difference in refractive index between the gas atmosphereand the glass. The reflected portion of the impinging total radiationintensity is generally about 4%. The non-reflected part of the infraredradiation propagates further in the transparent layer and a small partof it is scattered or absorbed. In one embodiment, the radiationcomponent transmitted here impinges on [a] layer of opaque quartz glass,which diffusely scatters and absorbs infrared radiation. The opacity ofthe layer of opaque quartz glass prevents the direct transmission of theinfrared radiation in favour of scattering and absorption. On the layerof opaque quartz glass, part of the infrared radiation is againreflected. The double reflection on the layer sequence of glass layerand layer of opaque quartz glass means that the non-reflected radiationcomponent is only absorbed within the infrared-radiation-absorbing layerof opaque quartz glass and generates heat there, whereas the hot gasatmosphere around the shear zone only affects the inside facing theblank by heat conduction. The heat input into the thermal radiationdissipator by radiation therefore takes place substantially in the layerof opaque quartz glass and thus at a different point than the heat inputby heat conduction. As a result, on the one hand the inside remains hotenough to bind SiO₂ deposits on the inside and vitrify them so that theydo not fall off, and on the other hand overheating of the inside isavoided. The opacity of the layer of opaque quartz glass is in oneembodiment caused by a porosity of the quartz glass in the range of 2 to8%.

The thermal radiation dissipator in one embodiment consists completelyof quartz glass and particularly in one embodiment of quartz glass thathas been produced synthetically from silicon-containing startingsubstances by pyrolysis or hydrolysis.

Definitions and Measuring Methods

Individual steps and terms in the above description as well as measuringmethods are additionally defined below. The definitions are part of thedescription of the embodiments. If there is a material contradictionbetween one of the following definitions and the rest of thedescription, the statements in the description are definitive.

Quartz Glass

Quartz glass here means glass with an SiO₂ content of at least 87 wt. %.It is undoped (SiO₂ content=100%) or it contains dopants, such as e.g.fluorine, chlorine or oxides of rare earth metals, aluminium ortitanium. A high-silica glass means a glass with an SiO₂ content of atleast 80 wt. %.

Porosity—Measuring the Pore Volume

The “pore volume” of a porous material refers to the free volume withinthe material occupied by voids. The pore volume is measured using e.g. aporosimeter, where a non-wetting liquid (such as e.g. mercury) ispressed into the pores of a porous material under the action of anexternal pressure against the opposing surface tension forces. The forceneeded is inversely proportional to the pore size and therefore, as wellas the total pore volume, the pore size distribution of the sample canalso be determined. Mercury porosimetry only detects pore sizes above 2nm (mesopores and macropores). “Micropores” are pores with pore sizes ofless than 2 nm. Their contribution to the porosity and to the specificsurface area is determined using the V-t method by nitrogen absorption,where a sample is held at different pressures and 77 K. The method isequivalent to the BET method, the pressure range being extended tohigher pressures so that surface areas of the non-microporous part ofthe material are also determined.

Transparency in the NIR Wavelength Range

For the wavelength range of the “near infrared” (abbreviated as NIR),there are different nomenclatures. Within the framework of thisapplication, in accordance with DIN 5031 part 7 (January 1984), it isdefined as the spectral range between 780 nm and 3000 nm.

Transparent in the NIR wavelength range refers here to a glass which,with a sample thickness of 10 mm, transmits at least 50% of theimpinging NIR radiation power.

Measuring the Concentration of Hydroxyl Groups (OH Groups)

The measurement takes place using the method of D. M. Dodd and D. B.Fraser, “Optical determination of OH in fused silica”, Journal ofApplied Physics, Vol. 37(1966), p. 3911.

Providing a Cylindrical Blank Composed of Doped Quartz Glass Example 1:Production by Gas Pressure Sintering

A cylindrical compact of SiO₂ pellets was fused in a gas pressuresintering process to form a component composed of the doped, transparentquartz glass. The gas pressure sintering process was performed in a gaspressure sintering furnace with an evacuable sintering mould composed ofgraphite with a cylindrical inner space. The mould was first heated tothe sintering temperature of 1700° C. while maintaining a negativepressure. Once the sintering temperature was reached, a positivepressure of 15 bar was established in the furnace and the mould was heldat this temperature for approx. 30 min. During the subsequent cooling toroom temperature, the positive pressure was further maintained until atemperature of 400° C. was reached. The quartz glass blank obtained hada diameter of 16 mm and a length of 100 mm.

Example 2: Production by Vapour Deposition

By outside deposition on a support body using the known OVD method, asoot body made of quartz glass was produced and this was then vitrifiedin a vacuum furnace. From the vitrified OVD cylinder a ⅙ longitudinalsegment was cut and this was rounded on a glass turning machine. Aquartz glass blank was obtained with a diameter of 80 mm which displayeda marked variation in refractive index across the diameter, causedsubstantially by the inhomogeneous distribution of the OH content.

Zone Melting Method Method Example (a)

The blank according to example 1 was then subjected to a zone meltingmethod (twisting). This processing operation is illustrated in a diagramin FIG. 1 . For this purpose, two support rods 3 were welded on to theend faces of the rod-shaped blank 1 using a plasma torch. The supportrods 3 were clamped in the spindles 6, 7 of a glass lathe. The spindles6; 7 define a working distance “D” of the glass lathe.

The glass lathe was equipped with an oxygen-hydrogen heating burner 2,which produced an oxyhydrogen flame 5.

The heating burner 2 was mounted on a displaceable carriage 11 and wasmoved thereon, by means of a drive, along the blank 1 which was clampedin the glass lathe (indicated in the Figure by the directional arrows8), the blank 1 being heated locally to over 2000° C. The impingementarea of the oxyhydrogen flame on the surface of the blank had a width ofabout 20 mm.

As a result of unequal rotational speeds (ω1=80 rpm, ω2=(−170) rpm) andopposite directions of rotation of the two glass lathe spindles 6, 7 ashear zone 9 formed in the heating region of the oxyhydrogen flame 5. Inthe shear zone 9, an intermixing and thus homogenizing of the glass tookplace. Its width B was smaller than the impingement area of theoxyhydrogen flame 5 and was about 5 mm. The shear zone 9 was moved alongthe longitudinal axis of the blank 10 by a reversing motion of theoxygen-hydrogen burner 2, intensively intermixing the rod-shaped blank 1along its entire length. In its movement along the longitudinal axis ofthe blank the shear zone 9 was limited at one side by a right-handcylindrical section 1.2 and at its other side by a left-hand cylindricalsection 1.1.

As soon as the soft shear zone 9 had formed, the two spindles 6, 7 ofthe glass lathe were offset radially to each other by about 2 mm suchthat the spindle's two axes of rotation 6.1 and 7.1 were parallel butnot coaxial to each other. Because of this radial offset, a deformationof the shear zone 9 was obtained in the direction of a non-circular,non-rotationally-symmetrical shape and a smaller width. This increasedthe homogenizing efficiency of the zone melting method.

The homogenized glass cylinder thus obtained had a diameter of about15.5 mm and a length of about 100 mm.

FIG. 2 illustrates a diagram of the corresponding rotation figure in aside view of the central axes 6.1 and 7.1: the first cylindrical section1.1 and the second cylindrical section 1.2 here are each rotated aroundtheir own axes of rotation, which run parallel to each other and whichare offset relative to each other by the said 2 mm in their verticalposition owing to the spindle offset. The axes of rotation correspond tothe central axes 6.1 (for the cylindrical section 1.1) and 7.1 (for thecylindrical section 1.2). The two cylindrical sections 1.1 and 1.2 thushave no common axis of rotation. In the shear zone 9 torsion is obtainednot only due to the different rotations of the cylindrical sections 1.1;1.2 but also due to the lateral offset of the two central axes 6.1 and7.1 relative to each other.

Method Example (b)

To homogenize the OH distribution, the blank according to example 2 waslikewise subjected to a zone melting method. The processing operationtook place using the device illustrated schematically in FIG. 1 with theadditional use of a tubular thermal radiation dissipator surrounding theshear zone 9. This had a length (dimension in the direction of thelongitudinal axis of the blank) of 300 mm; an inner diameter of 120 mmand a wall thickness of 27 mm. It was likewise mounted on the carriage11 and was moved synchronously with the heating burner 2 and with theaid of the same drive along the blank 1 clamped in the glass lathe. Thewall of the thermal radiation dissipator 70 had an opening 73, throughwhich the heating burner 2 or oxyhydrogen flame 5 projected. Between theblank 1 and the inner wall of the thermal radiation dissipator 70 anannular gap 12 remained, with an average gap width of 20 mm.

As a result of unequal rotational speeds (ω₁=(−40) rpm; ω2=120 rpm) andopposite directions of rotation of the two glass lathe spindles 6, 7 ashear zone 9 formed in the heating region of the oxyhydrogen flame 5. Inthe shear zone 9, an intermixing and thus homogenizing of the glass tookplace. Its width B was smaller than the impingement area of theoxyhydrogen flame 5 and was about 10 mm. The shear zone 9 was movedalong the longitudinal axis of the blank 10 by a reversing motion of theoxygen-hydrogen burner 2, intensively intermixing the rod-shaped blank 1along its entire length.

In its movement along the longitudinal axis of the blank the shear zone9 was limited at one side by a right-hand cylindrical section 1.2 and atits other side by a left-hand cylindrical section 1.1. As soon as thesoft shear zone 9 had formed, the two spindles 6, 7 of the glass lathewere offset radially to each other by about 2 mm, so that the spindle'stwo axes of rotation 6.1 and 7.1 were parallel but not coaxial to eachother. Because of this radial offset, a deformation of the shear zone 9was obtained in the direction of a non-circular,non-rotationally-symmetrical shape and a smaller width. This increasedthe homogenizing efficiency of the zone melting method.

In this way a glass cylinder with a diameter of about 79 mm wasobtained.

Method Example (c)—Comparative Example

To homogenize the OH distribution a blank 1 produced according toexample 2 was likewise subjected to a zone melting method, but withoutestablishing an offset between the spindle's two axes of rotation 6.1and 7.1; the axes of rotation 6.1 and 7.1 thus coincided. In thistechnique without a radial offset, the shear zone 9 was deformed in acircular or rotationally symmetrical fashion.

The diagram of FIG. 8 illustrates the difference in the radial hydroxylgroup distributions of the samples of method example (b) and thecomparative example (c). On the y-axis the hydroxyl group concentrationCox is plotted (in wt. ppm) against the radial position P (in mm), asmeasured in the homogenized glass rod. Curve (c) illustrates thatwithout a lateral offset a parabolic OH-group distribution in theconcentration range of 170 to 215 wt. ppm is maintained, as was alsomeasured in the original twisted rod before the thermal-mechanicalintermixing treatment. With the lateral offset of the axes of rotationaccording to method example (b), on the other hand, a distribution curve(b) is obtained which is flattened in the middle, having significantlylower OH variation in the concentration range between 180 and 190 wt.ppm.

FIG. 3 illustrates a diagram of the rotation figure in anothertechnique, which is produced by an off-centre arrangement of the supportrod 3 for the right-hand cylindrical section 1.2 in the spindle 7. Thecentral axis 7.1 of the cylindrical section 1.2 and/or its support rod 3here is offset by 3 mm from the machine axis of rotation 7.2 of thespindle 7. The first cylindrical section 1.1 thus rotates around thecentral axis 6.1 at the rotational speed ω₁ and the second cylindricalsection 1.2 is turned at the rotational speed ω₂ around a circular patharound the machine axis of rotation 7.2 of the spindle 7, which iscoaxial to the central axis 6.1. The second cylindrical section 1.2 heredoes not rotate around its own central axis 7.1 but describes a circularpath around the actual machine axis of rotation 7.2 and around thecentral axis 6.1 that is coaxial thereto.

In the shear zone 9 torsion is obtained not only due to differentrotations (ω₁; ω₂=0) of the cylindrical sections (1.1; 1.2) around theirrespective central axes (6.1, 7.1), but at the same time the cylindricalsection 1.2 that is offset relative to the machine axis of rotation 7.2of the spindle 7 describes a circular path around the common axis ofrotation (central axis 6.1). This results in particularly intensiveintermixing in the shear zone 9, not only in the tangential directionbut also in the radial and partially also in the axial direction of thelongitudinal axis of the blank.

In another technique for forming a non-rotationally-symmetrical, narrowshear zone with good intermixing even in the radial direction, the shearzone performs an alternating motion during transverse displacement alongthe longitudinal axis of the blank. FIG. 4 illustrates a diagram of acorresponding time-position diagram. On the y-axis the time t (in s) isplotted against the spatial position P of the heat source (in mm). Theaxial translational motion of a heat source along the longitudinal axisof the blank is superimposed with somewhat briefer reverse movements;some of the reversal points are marked by broken lines. During theforwards and backwards movement, the distances differ by a factor ofabout 2 and the rates of advance are equal.

Method Example (d)

In another technique for homogenizing the OH distribution, a furtherblank, having a radial hydroxyl group profile with a step shape (seeFIG. 9 ), was likewise subjected to a zone melting method using athermal radiation dissipator 10 (as described with reference to FIG. 1). The oscillating motion of the shear zone here was generated byrotating the first cylindrical section at a first rotational speed ω₁and the second cylindrical section at a second rotational speed ω₂,periodically varying the first and/or second rotational speed. Table 1gives parameters of the preferred exemplary embodiment:

TABLE 1 Variation Curve range Mean Amplitude Frequency Rotation shape[rpm] [rpm] [rpm] [min⁻¹] ω₁ Sinusoidal  40-160 100 60 1.2 ω₂ Sinusoidal(−160)-(−40) −100 60 1.2

The rotational speeds ω₁ and ω₂ changed periodically according to asinusoidal oscillation, the oscillation frequencies being of equal size,so that a constant phase shift of 200 rpm was obtained. The mean valuesand amplitudes differed, however, so that a periodic change in therotational speeds was obtained which led to an oscillating motion of theshear zone.

Method Example (e)—Comparative Example

To homogenize the OH distribution the quartz glass blank as mentioned inmethod example (d) with a step-shaped radial hydroxyl group distributionprofile was likewise subjected to a zone melting method, but without anoscillating motion of the shear zone as in method example (d). Therotational speeds of the spindle's axes of rotation 6.1 and 7.1 herewere adjusted to constant values of −20 rpm and +180 rpm respectivelyand the axes of rotation 6.1 and 7.1 thus coincided. In this techniquethe shear zone 9 was deformed in a circular or rotationally symmetricalfashion.

The diagram of FIG. 9 illustrates the difference in the radial hydroxylgroup distribution of the samples of method example (d) and comparativeexample (e). On the y-axis the hydroxyl group concentration C_(OH) (inwt. ppm) is plotted against the radial position P (in mm), as measuredin the homogenized glass rod. Curve (e) illustrates that, without anoscillating motion of the shear zone, an OH group distribution isobtained with a marked concentration maximum in the centre of the blankand in a concentration range of 200 to 270 wt. ppm. In contrast, curve(d) illustrates that with an oscillating motion of the shear zoneaccording to method example (d) a hydroxyl group concentration profileis obtained which is flattened in the middle and has a significantlysmaller OH variation in the concentration range of between 210 and 250wt. ppm.

FIG. 5 illustrates a diagram of a further technique for intensiveintermixing in the shear zone. The same reference numbers as in FIG. 1denote the same or equivalent components or parts as already explainedwith reference to FIG. 1 . In this technique the two central axes 6.1and 7.1 are arranged such that they are tilted relative to each other,so that they form a tilt angle α of about 165 degrees. The cylindricalsections 1.1 and 1.2 here are oriented such that the first central axis6.1 and the second central axis 7.1 extend downwards starting from theshear zone 9. If the vertical positions of the two glass-lathe spindles6, 7 remain constant during a twisting stroke (indicated by thedirectional arrow 8), the tilt angle varies continuously. In this methodvariant too, the thermal radiation dissipator 70 explained withreference to FIG. 1 can be used advantageously.

In order to keep the tilt angle α constant in the course of the zonemelting method, a vertical adjustment of the spindles 6, 7 (or of therespective chuck) is necessary, as indicated schematically in FIG. 6with reference to three process phases A, B and C. Starting from theprocess phase A with a comparatively short left-hand cylindrical section1.1 and a comparatively long right-hand cylindrical section 1.2, thecontinuous displacement of the heating burner 2 and the shear zone 9towards the right-hand glass lathe spindle 7 requires a continuousraising of the right-hand glass lathe spindle 7 to keep the tilt angle αconstant in the course of the zone melting method, as illustrated by thedirectional arrows 62. Instead of the continuous movement of one of thespindles (chucks), it is also possible to displace both (spindles)chucks in a vertical direction. To keep the length of the blankconstant, the chucks can be moved towards each other.

FIG. 7 illustrates a larger illustration of the thermal radiationdissipator 70 of FIG. 1 in a view of its end face. The wall consists oftwo coaxial and adjacent layers, specifically an inner layer 71 composedof synthetically produced quartz glass with a low bubble content and alayer thickness of 1.5 mm and an immediately adjacent outer layer 72composed of synthetically produced, opaque quartz glass with a layerthickness of 15 mm. The glassy inner layer 71 contains no visuallydiscernible pores. It reflects part of the impinging infrared radiation(about 4% of the total radiation intensity) and is otherwise transparentto infrared radiation over a broad wavelength range. The opacity of theadjacent outer layer 72 is produced by a porosity of about 5%. Theinfrared radiation impinging on the outer layer 72 is likewise partiallyreflected at the interface with the inner layer 71, but is predominantlyscattered and absorbed in the outer layer 72. Apart from an accessopening 73 for the heating burner 2, the wall is closed. The end facesare open.

In the zone melting method according to one embodiment, the thermalradiation dissipator 70 absorbs part of the heat energy from the shearzone 9 in particular by heat radiation and heat conduction, thus beingheated itself, and emits this energy as longer-wave infrared radiation.The thermal radiation dissipator 70 is arranged centrally to the shearzone 9 and projects beyond it at both ends, so that the emitted heatenergy is also transferred to the bulk of the glass adjacent to theshear zone 9. Compared to a zone melting method without the thermalradiation dissipator 70 the axial temperature gradient and thetemperature difference between the middle of the blank and its peripheryare reduced by the pre- and post-heating. A contribution to this is madeby the fact that the burner gases introduced through the central accessopening 23 into the gap 12 between thermal radiation dissipator 70 andblank 1 flow out at both ends of the shear zone 9 to the right and leftalong the longitudinal axis of the blank 10, thus heating the regionsnext to the shear zone 9.

As a result of this evening out of the temperature profile within theshear zone 9, the risk of cracking due to mechanical stresses isreduced.

The double reflection at the layer sequence of inner layer 71 and outerlayer 72 means that the non-reflected radiation component is onlyabsorbed in the infrared-radiation-absorbing outer layer 72 andgenerates heat there, whereas the hot gas atmosphere around the shearzone 9 only acts on the tubular inner wall of the thermal radiationdissipator 70 by heat conduction. The heat input into the thermalradiation dissipator 70 by radiation therefore takes place substantiallyin the outer layer 72 and thus at a different point to the heat input byheat conduction. As a result, on the one hand the inside remains hotenough to bind SiO₂ deposits on the inside and vitrify them so that theydo not fall off, and on the other hand an overheating of the inside isavoided.

The following table illustrates test parameters and results of zonemelting methods with a thermal radiation dissipator (Test 1) and withouta thermal radiation dissipator (Test 2).

TABLE 2 ω₁ ω₂ T_(max) v B Test [rpm] [rpm] [° C.] [mm/min] [mm] Cracks 1−60 +140 2290 10 5 No 2 −60 +140 2140 8 9 Yes

The terms in the table have the following meanings:

ω₁, ω₂: rotational speeds on either side of the shear zoneT_(max): maximum temperature in the region of the shear zonev: translational speed of heating burner and thermal radiationdissipatorB: maximum width of the shear zoneCracks: occurrence of a crack after completion of the zone meltingmethod.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments illustrated and describedwithout departing from the scope of the present embodiments. Thisapplication is intended to cover any adaptations or variations of thespecific embodiments discussed herein. Therefore, it is intended thatthese embodiments be limited only by the claims and the equivalentsthereof.

1. A method for homogenising glass, comprising: (a) providing acylindrical blank composed of the glass having a cylindrical outersurface that extends along a longitudinal axis of the cylindrical blankover a length of the cylindrical blank between a first end face and asecond end face, (b) forming a shear zone in the cylindrical blank bysoftening a longitudinal section of the cylindrical blank and subjectingit to a thermal-mechanical intermixing treatment, and (c) displacing theshear zone along the longitudinal axis of the blank, wherein thedisplacement of the shear zone along the longitudinal axis of thecylindrical blank is superimposed by a simultaneous oscillating motionof the shear zone along the longitudinal axis of the blank, and whereinthe first end of the cylindrical blank is rotated at a first rotationalspeed and the second end of the cylindrical blank is rotated at a secondrotational speed, wherein an oscillating motion of the shear zone isgenerated by periodically varying the first and/or second rotationalspeed.
 2. The method according to claim 1, wherein the displacement ofthe shear zone along the longitudinal axis of the blank is caused bylinear axial translational movement of a heat source along thelongitudinal axis of the blank, wherein an oscillating motion of theshear zone is generated by superimposing a reversing motion of the heatsource on the translational movement.
 3. The method according to claim1, wherein the displacement of the shear zone along the longitudinalaxis of the blank is caused by linear axial translational movement of aheat source along the longitudinal axis of the blank, wherein anoscillating motion of the shear zone is generated by superimposing areversing motion of the heat source on the translational movement. 4.The method according to claim 1, wherein the blank has a diameter D andthat the shear zone has a width that is less than 0.3×D.
 5. The methodaccording to claim 1, wherein the shear zone is at least partlysurrounded by a thermal radiation dissipator, a lateral dimension ofwhich, in the direction of the longitudinal axis of the blank, isgreater than the shear zone and smaller than the length of the blank,wherein the thermal radiation dissipator is moved along the longitudinalaxis of the cylindrical blank synchronously with the shear zone.